Hydrogen Assisted Remote Plasma Enhanced Chemical Vapor Deposition of Amorphous Silicon Nitride Films
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ABSTRACT We show that in the RPECVD process the addition of a small upstream flow of H2 gas, combined with the He injected into the plasma region of the reactor, promotes chemical exchange between H and D when the material was deposited from deuterated Silane and ammonia and densifies the aSi:N:H network. We have deposited hydrogenated stoichiometric nitrides with and without H2 injected into the plasma region. For small H2 flows, the deuterated samples show Fourier transform infrared (FTIR) N-D and N-H stretching and bending modes that interchange their populations, as the total bonded hydrogen content in the films remain constant. The hydrogen assisted grown films show, for small H2 flows, a constant amount of bonded-H, an optical index of refraction n= 1.85, and reduced etch rate. 1. INTRODUCTION This work continues the[l] study of the addition of a small flow of H2 gas combined with the He injected into the plasma region of the reactor while growing amorphous silicon nitrides films. Here, we have used Fourier Transform Infrared Spectrometry (FTIR) to measure the dependence of stretch modes as a function of the flow of H2 added upstream into the Remote Plasma Enhanced Chemical Vapor Deposition (RPECVD) chamber [2]. Nitride layer chemical bonding configurations affect the electrical performance of MIS devices such as capacitors and thin film transistors [3]. In a-Si:N:H the existence of bonded-H as Si-H and SiN-H groups and the possibility of hydrogen evolution by post deposition treatments as Rapid Thermal Annealing
(RTA) influence the electrical characteristics of devices [4]. To reduce the amount of bonded-H in amorphous silicon nitride layers of stack structures, one often resorts to post deposition high temperature processing. Post deposition heat treatments, however, can cause a compromising diffusion of atoms through the different neighboring film layers. It is therefore of importance to find other options, rather than post deposition heat treatment, for controlling bond arrangements within deposited thin films. We present that for small H2 flows, the samples show etch rate characteristics [4] of samples that underwent RTA treatment, while the amount of bonded-H is kept constant. The film densifies but there is no appreciable increase in the magnitude of the optical index of refraction. Here, we show that films grown by remote PECVD from Silane and ammonia mixtures and the assistence of a small flows of H2 densify, and may not require post deposition heat treatment. We propose that one can benefit by having small flows of H2 into the plasma region, as the hydrogen bonding does not increase for small H 2 flows, while improving the Si-N network resistance to etchants. The samples within this study have been deposited from deuterated ammonia and deuterated Silane source gases. The ratio of ND3 to SiD4 has been adjusted so that deposited films contained FTIR detectable bonded-D in SiN-D arrangements, but not in Si-D arrangements. Once H2 is introduced as a plasma gas, the films show both SiN-H and SiN-D bonds
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